1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus. Further, this invention relates to an image heating device for fixing an unfixed image that is used in such an image forming apparatus and employs an electromagnetic induction heating method.
2. Related Background Art
JP2001-60490 A discloses an image heating device employing the electromagnetic induction heating method.
FIG. 22 is a cross sectional view of the image heating device disclosed in JP2001-60490 A.
In FIG. 22, reference numeral 901 denotes a heating tube that generates heat by induction heating. The heating tube 901 is fitted on an outer periphery of a guide 907 in the shape of a cylindrical tube and supported rotatably. Further, reference numeral 902 denotes a pressure roller that makes contact under pressure with the heating tube 901. A recording paper sheet 908 is passed through a nip part (pressure-contact part) between the heating tube 901 and the pressure roller 902, so that an unfixed toner image formed on the recording paper sheet 908 is fixed thermally. Further, reference numeral 904 denotes an excitation coil that is disposed in an inner portion of the guide 907 and generates a high-frequency magnetic field. Reference numeral 905 denotes a core that is disposed at a winding center of the excitation coil 904, and reference numeral 906 denotes a cancel coil that is wound on an outer periphery of the core 905 and regulates an amount of heat generated.
The recording paper sheet 908 carrying the unfixed toner image is conveyed to the nip part in a direction indicated by an arrow 910 in FIG. 22. Using the heat of the heating tube 901 and the pressure exerted between the heating tube 901 and the pressing roller 902, a fixed toner image is formed on the recording paper sheet 908.
FIG. 23 is a plan view of the cancel coil 906 provided in the image heating device shown in FIG. 22 as seen from a direction indicated by an arrow S. A lateral direction in a plane of FIG. 23 corresponds to a rotation axis direction of the heating tube 901 shown in FIG. 22, which corresponds to a width direction of a recording paper sheet that is passed.
As shown in FIG. 23, loop portions 906a and 906b of the cancel coil 906 are positioned so as to correspond to both ends of a recording paper sheet that is passed. A switching element 909 is connected across the cancel coil 906 so as to pass/interrupt an electric current to the cancel coil 906.
When a recording paper sheet of a width smaller than a length of an effective portion of the heating tube 901 is passed, the cancel coil 906 is short-circuited. Therefore, by a change in magnetic flux that is generated in the core 905 by the excitation coil 904, an induction electromotive force is generated in the loop portions 906a and 906b of the cancel coil 906, and thus an induced current is generated in the cancel coil 906. In FIG. 23, arrows indicate the directions of an induced current generated in the cancel coil 906 at a particular moment. This induced current causes a magnetic flux to be generated in the loop portions 906a and 906b, which is in a direction opposite to that of the magnetic flux generated in the core 905 by the excitation coil 904. Thus, an amount of heat generated in both end portions of the heating tube 901 is suppressed.
On the other hand, in the case where a large-width recording paper sheet is passed, the cancel coil 906 is opened. Therefore, an induced current is not generated in the cancel coil 906, and thus an amount of heat generated in both the end portions of the heating tube 901 is not suppressed.
As described above, a distribution of an amount of heat generated in the width direction can be regulated so as to correspond to a width of a recording paper sheet.
However, the image heating device disclosed in JP2001-60490 A presents the following problems.
In this configuration, the cancel coil 906 needs to be wound so as to form the loop portions 906a and 906b at both the end portions in the width direction, while the loop portions 906a and 906b away from each other at both ends need to be coupled. Further, the complexity of the shape requires complicated winding work. For reasons including the above-mentioned reasons, this configuration results in a costly device. Moreover, when the cancel coil 906 is brought into conduction, magnetic flux generated by the excitation coil 904 does not pass through the core 905 and thus is diffused into a wide area, thereby causing unintended heating of a constituent member.